Hibernate ORM and Jakarta Persistence

Problem -> anti-pattern -> fix library for entity mapping and querying. Framework-neutral: it covers EntityManager/Session behavior, JPQL/HQL, and mapping annotations. If the project uses Spring Boot and the java-springboot and spring-boot-testing skills are installed, use them for Spring Data repositories, transaction wiring, and @DataJpaTest; for Panache and reactive persistence see the Quarkus skills. Integration tests against a real database: java-testcontainers.

First: detect the stack

1. Read pom.xml/build.gradle (or the dependency tree) for hibernate-core and the persistence API: jakarta.persistence (Hibernate 6+/Jakarta EE 9+) vs javax.persistence (Hibernate 5, see javax-to-jakarta-migration).
2. Note the Hibernate major version; behavior differs (see references/tooling-and-versions.md). Hibernate 6.0-6.1 implement Jakarta Persistence 3.0, 6.2+ implement 3.1, 7.x implements 3.2, and Hibernate 7 needs Java 17+.
3. Note the database and whether the schema is managed by Flyway/Liquibase.
4. Turn on SQL logging and statistics in a test (see references/tooling-and-versions.md) before "optimizing" anything: measure statement counts.

Quick decision table

Symptom	Likely cause	Go to
Many similar SELECTs for one request	N+1 on lazy association	references/fetching-and-n-plus-one.md
LazyInitializationException	Lazy access outside session	references/fetching-and-n-plus-one.md
HHH000104 / HHH90003004 warning, big memory use	Pagination with collection fetch join	references/queries-pagination-batching.md
Inserts slow, no JDBC batching	IDENTITY ids or batching not configured	references/identifiers-and-entity-design.md, references/queries-pagination-batching.md
Duplicate rows / lost children / Set anomalies	Bad equals/hashCode or cascade misuse	references/identifiers-and-entity-design.md, references/associations-and-cascade.md
Lost updates, deadlocks	Missing locking strategy	references/locking-transactions-caching.md
Stale or surprising data with caches	Second-level/query cache misuse	references/locking-transactions-caching.md
Schema drift, ddl-auto in prod	No migration tool	references/tooling-and-versions.md

Top rules (apply during review)

1. Associations: @ManyToOne/@OneToOne default to EAGER; declare fetch = FetchType.LAZY explicitly. @OneToMany/@ManyToMany are LAZY by default and should stay so. Choose fetching per query (JOIN FETCH, entity graph), not by making mappings EAGER.
2. Prefer @ManyToOne owning side plus a bidirectional @OneToMany(mappedBy=...) only when needed; keep both sides in sync through helper methods.
3. Set or List for collections? Use Set for @ManyToMany; a unidirectional @OneToMany with @JoinColumn or List without an order has costly update patterns.
4. Never use CascadeType.REMOVE/ALL on @ManyToOne or @ManyToMany. Use orphanRemoval only on a true parent-owned child collection.
5. Ids: SEQUENCE (pooled optimizer, allocationSize matching the DB increment) for generated numeric ids; IDENTITY disables JDBC insert batching; use UUIDs (time-ordered preferred) when ids must be generated client-side.
6. Entities: no Lombok @Data/@EqualsAndHashCode/@ToString over associations; implement equals/hashCode deliberately.
7. Return DTOs/projections from read endpoints rather than entities; never expose entities (lazy proxies) directly in an API.
8. Paginate with a stable ORDER BY including a unique tiebreaker; never combine setMaxResults with a collection JOIN FETCH.
9. Keep transactions short, at the service boundary; no remote calls or user interaction inside one.
10. Write and flush in batches for bulk work and clear the persistence context periodically.
11. Verify behavior with statement counts in tests against the real database engine.
12. Follow the schema management rule below.

Schema management rule

This is the single rule other skills refer to by name:

• Prototype/local: hibernate.hbm2ddl.auto=update (or create-drop for throwaway tests) is acceptable.
• Production (default): versioned migrations with Flyway or Liquibase, and hibernate.hbm2ddl.auto=validate (or none). Add them when the user wants versioned migrations or the app is headed to production.
• Externally managed schema: none (or validate); do not add a migration tool.

Review focus

Check statement counts for N+1, explicit LAZY on to-one associations, sequence-based ids with matching allocationSize, and equals/hashCode stable across transient to persistent. Version-dependent details (Hibernate 5/6/7): references/tooling-and-versions.md.

References

• references/fetching-and-n-plus-one.md: N+1 detection and fixes, lazy vs eager, LazyInitializationException, open-session-in-view, DTO projections.
• references/identifiers-and-entity-design.md: id generation, equals/hashCode, Lombok, records vs entities, enums and converters, value types.
• references/associations-and-cascade.md: bidirectional helpers, cascade, orphanRemoval, collection choice, @ManyToMany.
• references/queries-pagination-batching.md: pagination (offset, keyset, in-memory warning), bulk operations, JDBC batching, StatelessSession.
• references/locking-transactions-caching.md: optimistic/pessimistic locking, transaction boundaries, flush modes, second-level cache.
• references/tooling-and-versions.md: SQL logging, statistics, tests, connection pool sizing, migrations, Hibernate 5/6/7 differences.

Canonical docs: Hibernate ORM user guide https://docs.hibernate.org/orm/current/userguide/html_single/ and Jakarta Persistence https://jakarta.ee/specifications/persistence/. Deep background articles: https://vladmihalcea.com/.

Upgrading (fetch the page for the target version; prefer it over this skill): migration guides https://hibernate.org/orm/documentation/migrate/ (6.6: https://docs.hibernate.org/orm/6.6/migration-guide/, 7.0: https://docs.hibernate.org/orm/7.0/migration-guide/), release notes https://hibernate.org/orm/releases/.